1. Field of the Invention
The present invention relates to a piezoelectric material, a piezoelectric device, and a method of producing the piezoelectric device, in particular, a lead-free piezoelectric device whose insulation property is improved by the promotion of crystallization under an excess oxygen atmosphere.
2. Description of the Related Art
A piezoelectric device is typically formed of a lower electrode and an upper electrode, and a bulk-shaped or film-shaped piezoelectric material interposed between those electrodes. As a piezoelectric material, lead-based ceramics such as lead zirconate titanate having a perovskite-type structure (hereinafter, referred to as “PZT”) is generally used.
However, PZT contains lead at the A-site of a perovskite skeleton. Therefore, the effect of the lead component on the environment is considered to be a problem. In order to respond to this problem, a piezoelectric material using a lead-free perovskite-type metal oxide has been proposed.
As a typical lead-free piezoelectric material, there is exemplified BiFeO3 (hereinafter referred to as “BFO”), which is a perovskite-type metal oxide.
For example, Japanese Patent Application Laid-Open No. 2007-287739 discloses a BFO-based material containing lanthanum at its A-site. BFO is a good ferroelectric, and it has been reported that the amount of remanent polarization of BFO measured at a low temperature is high, and thus the material is expected to have good properties as a piezoelectric material. However, there is a problem in that the voltage to be applied to BFO for producing piezoelectric distortion is limited, because the insulation property of BFO in a room-temperature environment is insufficient.
In view of the foregoing, Japanese Patent Application Laid-Open No. 2007-221066 proposes a method of suppressing an increase in leakage current, the method involving substituting iron as the B-site element of BFO with manganese to improve the insulation property. However, BFO whose iron atoms are partially substituted with manganese itself suffers from the problem that its piezoelectric constant is too low.
Japanese Patent Application Laid-Open No. 2005-011931 discloses an approach to substituting the B site of BFO with Co at a ratio of 1 to 10% as an attempt at improving the ferroelectric property of a memory device using BFO. In a conventional method of producing a BFO thin film, however, the amount of undesired phases (hereinafter, referred to as “secondary phases”) which are not of perovskite-type structures increases as substitution with Co progresses. Accordingly, a problem arises, viz., a reduction in the material's insulation property.
The present invention has been made with a view to cope with such problems, and an object of the present invention is to provide a piezoelectric material whose insulation property is improved while its performance as a piezoelectric material is not impaired.
Another object of the present invention is to provide a piezoelectric device using the piezoelectric material and a method of producing the piezoelectric device.